Radar ground approach system



` Oct. 13, 1953 J. N. MARSHALL 2,655,650

RADAR GROUND APPROACH SYSTEM Filed March 19, 1951 5 Sheets-Sheefv l TAW/V5.

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y f lNvl-:NToR fl/ halfway ima ATTORNEY Oct. 13, 1953 J, N, MARSHALL 2,655,650

RADAR GROUND APPROACH SYSTEM A INVENTOR ATTORNEY Oc. 13, 1953 J. N. MARSHALL 2,655,650

RADAR GROUND APPROACH SYSTEM Filed March 19, 1951 5 Sheets-Shea*v 3 ATTORNEY Patented Oct. 13, 1953 UNITED STATES? PATENT OFFICE 2,655,650*- RADAR GRoUNnAPPRoAoH SYSTEM John MarshalLBryn Mawr, Pai, ass'ignor to Radio Corporation' of- America; a corporationv` of Delaware llrfllClaifns. Yl The present invention relates-to radar (radio echov detection and; ranging and' more partici-1'-V larly to radar ground' approach systems for" air? crafts. g In order' to simplify theioperationof ground approach control of aircraft' about' toland at a landing field, it is" desirable' toprovidethe azimuth, elevation, and range inforrnation"A all' in ai single Visual display. Af composite displayand means for producingit are described'and `claimed in' the UQ S. Patent'2,459';4'81 to'Irving Wolff-and Philip J. Herbst and'entitledlInstrumentlanding System. An improved system' of the*'sanie type' is described' in the copendingapplication; Serial No. 783,354, filed October 31 1947;. of Harold J Schrader', now'P'atent'2','549,641l In a display for Ground Control'Approach (GCA) radar ofithe type' described in'the mentioned patent'` or application; a' vi'sualsp'ot representing the aircraft is shownon'ascreen; the spot being displaced to the '1ight"'or"lft' ol va 'line 'representing the runway' by 'anamount' indicative'of 'the azimuthal error frorrr'thed'esired"glidepath' A horizontal line indicates' the' elevation" and" is above or' below' the' aircraft spot' b'y.' an" amount indicative ofthe angleb'y' which' th'e aircraft' is above or below thedesired"glidepath. One o'f the diiculties encounteredfwith" thesesystems has been that the horizontal line"described` above has a substantialwidth, because producedv from a displayed spot or the signals corresponding thereto. For example, .the line may. be produced by impressing a horizontal'repetitivesweep `on"a cathode ray tube atfthe-.tirnesignals correspond` ing to range and `elevation,arevloeingfimpressed on the tube. Butsuch--signals cause a-v spot*A or line of nite Width. This width makes it difl cult for the pilot or Vobserver to judge -accurately the amount of v elevation error with respect-to the glide path. Judging this error isparticularly difficult when the horizontal line passes through or in the near neighborhood of the aircraft spot. The line tends to obscure the-spot'and the relative positions of the spot and linearediicult or impossible of determination. A

It is an object Vof thepresentinvention'vto im prove a. displayfor GCA radar andjparticularly to improve a display of the ytype described A further object of. the .invention isto reduce the Width of the horizontal Yline in a display of the type described the positionof Which'lin'e "relai tive to the aircraftV spot is indicative of the'error in elevation with respect to the desirdglde path.

A-further object' of-*thefinvention'is tto=provide a-GCA Systen th-signals 01" Whih may be readily televised toa pilot4v or`- other remote operator'.

The foregoing andi other' objects, advantages, and novel features off the'invention Will be more apparent ffroiri`I the' following description when takenin connection? with' the' accompanying dravvfing Wh'i'ch like 'reference numerals refer mi like partsla'rid-irilwhicu'r- Fig'. 1-Y is a' Schelr'iatie4v lolo'irll diagram' 0f an embodiment? of the presentY invention;

Fig. 5 isaschein'atiev W? of ytli"Stora'ge target ora-storage tubejofFig to aidin explaining the-operation ofvtle-invetion In accordance 'with the invention,-

a display offthe' type describedt onlyfa single horizontal sweep* line" is briglte dfi tof produce* ar" sharp'. easily-'read'andiobseved horizon lineI on the display tube". The observer"A or pilot can then judgev accurately" his' eleva-tion above or'- below the proper'g-lide-'path by 'the-distance yofthe line aboveA or below'y the;A pip4I- corresponding' to the range'and azimuth 'position oftheaircraft. The breadth' 'of `the" horizontal l'line d'oes'fn'ot obscure tl'iel accuracy" ofithe observation.' An important feature of the invention'rsid'e's'in`th'ejpro'ducing of "'ae 'single pulse'fou'rring atY a constant predeterinin'edtim'e after'a 'seriesof pulses'de'rived from a spot stored storalgetub'e'. Such' a' 'stored spot' produces af'sries'ofpulses in' the video signais" as "the storedspot israd'." These"ser'ies"of p`1"sS` a'r V slfrtcle'd' pi:du-f'a/` 10'1f1'g`si`1f1g1ey pls'f Frihtflellg i''gl pulse'a short single pil'se 'is derived' frialonostable pulse'` circuit'. The derived 'pulsebursa'or'moe accurately has its in'eptioat, a"pdetrn`1ined time" after theavra'ge timev of thelon'g pulse, or' what is the" saniey a predetermined time after'the Inean time of the series of puls's'." As"mei'fily'lappears hereinafter; `the 'deidlpl's I riay'beebployed to produce" the' qs'ira nafrw'iin'e 3 for storing in separate azimuth and elevation zones on the storage target the information secured by the radar. In the azimuth zone is stored a spot whose position in respect to the undeected position of a cathode ray beam controlling the storage corresponds to the position of the aircraft in range and azimuth. In the elevation zone is stored a spot whose position along a line with respect to the undeflected cathode ray position corresponds to range plus a quantity proportional to the elevation error, that is the elevation angle of said aircraft from the elevation angle of the glide path at the aircraft range. Signals corresponding to those stored in the zones are taken in separate elevation and azimuth channels. In the elevation channel a a monostable pulse circuit is provided which produces from the signals arising from the stored elevation spot a single pulse of a predetermined time duration substantially equal to that of a single sweep in the television display to be reproduced. This single pulse is mixed with the signals from the azimuth channel to produce signals for the desired display. However, it is apparent that this single pulse, because 'of its speciiied time duration, produces a single comparatively narrow width line corresponding, in its displacement from the aircraft spot, the elevation error from the glide path when the display is reproduced on a cathode ray tube with a luminescent screen or the like. Other monostable pulse circuits are known which may be employed instead of the multivibrators shown and described herein. With respect to these other monostable circuits many of these may be those described in Volume 19, of Radiation Laboratory Series, entitled Wave Forms, edited by Chance et al. Furthermore, in accordance with the invention, the single pulse signal for the horizontal line is produced at a fixed or constant time after the mean time of occurrence of the first and last of signals in the elevation channel corresponding to the stored spot. A D. C. compensating voltage is then applied to restore the horizontal line signal to its correct time relationship, the said D. C. signal being applied preferably at the storage tube, whereby the horizontal display line when the aircraft is on the proper glide path passes through the center of the aircraft spot. In other words, in the final display the occurrence of the line corresponds to the center point of the elevation spot or elevation line of previous displays of this nature.

Referring now more particularly to Fig. l, the radar equipment includes a pulse transmitter I, a receiver 3, and in the present example two antennas 5 and 1. The antenna 5 has a fanshaped directive pattern, extremely narrow (perhaps a fraction of a degree wide) in azimuth, and relatively wide, say several degrees, in elevation. The antenna 1 has a similarly shaped pattern, narrow in elevation and wider in azimuth. The directive pattern of the antenna 5 can be oscillated throughout a sector of several degrees in azimuth and that of the antenna 1 can be oscillated similarly in elevation.

The antennas 5 and 1 may be structures of the type described in U. S. Patent 2,504,333, issued April 18, 1950, to Harley Iams, and entitled Radio Wave Devices. However, any other known antenna devices providing fan-like beams which can be oscillated in the manner described may be used. A motor 9 is coupled to the antennas 5 and 1 for driving them to scan cyclically through horizontal and Vertical sectors respect tively. The antennas are positioned so that the sectors which they scan include and are approximately centered on the glide path to be followed by an aircraft in landing.

The motor 9 drives a cam II, which periodically actuates a switch I3 to bring the antennas 5 and 1 into operation alternately, each completing one scan during its respective period of operation. A T-R box I5 is included in the connections between the switch I3 and the transmitter and receiver, to enable each of the antennas to be used for both transmission and reception.

A pulse generator' I1 modulates the transmitter I, and also controls a range sweep generator I9, which is an oscillator providing an output of substantially sawtooth wave form. A wave from the generator I9 starts coincidentally with the production of each pulse by the pulse generator I1.

A cathode ray storage tube 2l is provided which may be, for example, of the type described by Louis Pensak in the RCA Review, volume X, No. l, March, 1949, page 59, in the article entitled The Graphecon, a Picture Storage Tube. The storage tube 2l has a beam deflection yoke 23 for the writing beam connected to vertical and horizontal deflection signal amplifiers 25 and 21 respectively. The storage control electrode of the tube 2| is connected to the receiver 3. Conventional centering control means, not Shown, are provided for adjustably biasing the deflection means to position the cathode ray beam to a predetermined spot on the storage target 49 of the tube 2I.

Vertical deiiection signals are supplied to the amplier 25 through a mixer 29. The mixer 29 is connected directly to the range sweep generator I9, and through a switch 3| to an elevation sweep signal generator, which in the present example comprises a voltage divider 33 energized by a D. C. source 95, which may be raised or lowered to secure a fixed D. C. voltage compensation as by a divider 35a for a reason disclosed more fully hereinafter, to secure proper correspondence of horizontal line and aircraft spot in the final display. The divider 33 is driven continuously by the scanning motor 9. The resistance-rotation characteristic of the voltage divider 33 is uniform, so that the elevation sweep signal is substantially a sawtooth shaped voltage wave whose period is the same as the antenna scanning period. The switch 3i is ganged with and operates in synchronism with the switch I3, connecting the voltage divider 33 to the mixer 29 only during the periods when the elevation scanning antenna 1 is in operation.

The horizontal deflection amplifier 21 is connected to a switch 31 which also operates in synchronism with the switch i3. During azimuth scanning, the amplifier 21 is connected to a voltage divider 39, which is supplied by the range sweep generator E9 and is driven continuously by the scanning motor 9. The resistance-rotation characteristic of the rvoltage divider 39 is uniform like that ofthe voltage divider 33. Thus the range sweep signal as applied tc the amplifier 21 is modulated sawtooth fashion, the modulation period coinciding with the antenna sweep period. The switch 31 connects horizontal deflection amplifier 21 to another adjustable voltage divider 36 during elevation scanning by the antenna 1.

The operation of the system thus far described is as follows: WithA the switches I3, 3I, and 31 7 the antenna beams, the signals from only one stored spot are fed through this channel, and these will consist of a series of pulses one for each time the sweep passes vthrough the stored elevation spot For the remainder of the time no signal is produced from the elevation zone. These signals or pulses are stretched to produce a single pulse exemplified by the pulse in line c for each stored elevation spot. This single pulse is dierentiated as illustrated in Fig. 7, line d. It will be understood that Figs. 6 and 7 have widely different time bases. The output of differentiator 'i4 having the wave form similar to that of line d in Fig. 'I is applied to a timing voltage circuit the output of which is exemplified in line e of Fig. '7. This timing voltage circuit has the peculiar characteristic that its output Will reach a voltage V a xed period of time after the average time of occurrence of a pair of positive and negative going pulses, that is a xed time after the instant of time half Way between the times of occurrence of a pair of the output pulses arising from the difierentiated single pulse. rhe output of the timing voltage circuit 16 is applied to the threshold trigger circuit 'i8 which is actuated by the voltage V to produce a pulse having a time duration substantially equal to a single sweep, by which is meant a horizontal sweep (as described) oi the sweep circuits 64. The azimuth picture signals amplified in the amplifier 32 and the pulses from the threshold trigger circuit liti are mixed in a video mixer 84 for application to the television transmitter 86. If desired, signals from a ilying spot camera and amplier 83 may .also be mixed in the video mixer 84. A tele- 'vision receiver picks up the signal from the trans- :mitter 86 and transforms the signals mixed in the video mixer and transmitted by the transmitter 86 into a television picture on a cathode ray tube 02 having a luminescent screen.

Returning again tothe elevation channel 58, the pulse stretcher 12 is exemplified by the circuit of Fig. 2. The multivibrator 94 may be assumed to receive its input from the elevation signal amplifier which may be assumed to invert the signals from the storage tube output if they are of positive polarity or to leave them the same polarity if they are of negative polarity. Accordingly, the pulses exempliiied by line b of Fig. 6 are applied to the pulse stretched multivibrator S4 in negative polarity. This multivibrator 94 is a monostable multivibrator having only one condition of stable equilibrium to which it returns. The negative-going pulses cause the monostable multivibrator 94 to depart from its condition of stable equilibrium for a predetermined time which is preferably made slightly less than the time duration of one sweep period. Thus for the majority of time in which the multivibrator receives the pulses arising from a single stored spot in the elevation zone such as spot 5|, the pulse stretching multivibrator 34 is kept in one condition. Thus for each distinct stored spot in the elevation zone such as spot 5|, the output of the multivibrator becomes" a stretched single pulse with short drops to zero. This output is applied to an integrating amplifier 95 which largely eliminates the short drops. Remaining irregularities in the output as a result of the incomplete smoothing by the integrating amplier and of the input pulses from the multivibrator 04 are unwanted in the output. Therefore, the output pulse is preferably further shaped before leaving the pulse stretcher 12 by a clipper stage 96. The differ:-v entiator 14 of the elevation channel 58 is simply a resistor capacitor combination at the output of the clipper 96.

The output of the differentiator 14 is applied to the timing voltage circuit 15 which is exemplied by the circuit of Fig. 3, to which is applied the output pulse from diierentiator 14. The pulse is applied to a pair of monostable multivibrators |00, |02 each consisting of a pair of tubes |04, |06, and |08, ||0 respectively. The monostable multivibrator |00 is connected so that the tube |04 to the grid |05 of which the incoming signal is applied, is normally conducting. The monostable multivibrator |02 is connected so that the tube |08 which receives the input pulses is normally non-conducting. The applied pulses are of the form exemplified at line d of Fig. '7. The rst of each pair of pulses is positive and causes the tube |08 to start conducting. The next of the pair of pulses is negative and causes the tube |04 to cease conduction. A storage capacitor ||2 is connected through a pair of resistors ||4, H6, respectively, to the anodes IIS, |20 of tubes ||0 and |04. Let T1 and T2 represent the times of application of the respective pulses of a single pair. Then as long as the voltage charge on the storage capacitor ||2 buids up in a Substantially linear way, the voltage across the capacitor I2 at a time T may be written as:

where a is a constant for the circuit and voltages applied. This increasing voltage will reach a voltage V when V is equal to the above expression. That is, at a time T after the rst pulse of the pair when Therefore, the voltage across the capacitor reaches .the voltage V at a time after the mean time of occurrence or the average time of occurrence of the differentiated pair of pulses. It is clear that in any single frame scan over the storage target 40 the time T will occur substantially half way between the occurrence of the first and last of the pulses shown at line b of Fig. 6 which correspond to the elevation spot 5|. The timing voltage circuit mul tivibrators are preferably designed to have a recovery time to occur shortly after the voltage V is reached for the greatest time separation expected between T1 and T2, so that more than one stored elevation pulse may be readily handled by the system of the invention.

The output of the timing voltage circuit '16, which is the voltage across capacitor H2 is applied to the threshhold trigger circuit 18 of Fig. 4. The threshold trigger circuit 'I8 is another monostable multivibrator comprising a pair of tubes |22 and |24. This circuit has the tube |24 normally conducting and the tube |22 normally non-conducting. As the voltage from the timing voltage circuit 'i6 is applied, the tube |22 starts to conduct when the applied voltage reaches the threshold value V. The threshold trigger circuit 18 then produces an output pulse such as illustrated at line f in Fig. 7 through the cathode follower |26. The output pulse is preferably of the length of a single line of the sweep generated in the sweep circuits w b eht line, hoying a width en y 'to thatg' o v le 15er volte-'ee 'by Way' of e12- Correoo iolfaee io' the vertical oee'etioh 'eroplier 29 o" Fie l; or for exemple by S111 ,@blY ile me 'Roteotometer '35a of Fi'e- 11 Thee 'there' 'ere either' ieee-rie of dieoleoine .one or the other olf th' etereo- -elevetio'o soot @od the Stored azimuth 'l moein elevationtorprodueeineeio eleyetoo zo e b* e Stored Snot Whose position e-,looe e .line wie eoeot to Seid undeeered position. corresponde o said renee' plue a quantity proportional to the eleyartion angle of .Seid eroraft ,from the feleveoo angle of seid glide path, mee-.ne for den e .lo Separate azimuth elevation ohannele vmene-.1s corresponding respectively to, those stored in said genes, seid ^ohaumel signa-1s beine correlated with sweep signals for display on a cathode ray Lube, means in said 'elevation channel comprisinga monostvable pulse circuit for producing ,a'single pulse signa-1 of predetermined 'time duration Sub,- stantially equal to that of a single' one of 'said sweep signals and corresponding't said stored elevatidn spot-and 'means to mix the said`sin`g1e predetermined 'time duration pulse and`sig`na1S from said azimuth 'channel ih time felatinishilp, whereby the said mixed signalscorr'e'spohdi'tb thse of a cathode'ray tube 'display' in Whh visible "Spt ha'sapositionwth 'es'pect'fco'jn uril deected Cathode 4jry `tube Spot` gSitin fce` sporlding to thepositionof'slid airr'atn 'ira'nge vand azimuth' 'with respect' fc'eaid giohdt'at', and a Vijs'vibrle line in S'aid display res',ultugjj''fpih said single'predetemined tiiiie'duratio' p'uIs'e'S dis'tat lfi'bni .a'id'uideeeied pofpsiiili Tajr agrhunt 'cbrrespdig to" saidjarge plus 'ugil tit? proportial t`o` devaoiori fsaid 'eleic'iqii 11:11" eepeet .tof the 11 each other on said target, means controlling said cathode ray in response to said radar means during scanning in azimuth to produce in said azimuth zone a stored spot whose position with respect to the undeflected position of said ray corresponds to the position of said aircraft in range and azimuth with respect to said ground station, and means controlling said cathode ray during scanning in elevation to produce in said elevation zone a stored spot whose position along a line with respect to said undeflected position corresponds to said range plus a quantity proportional to the elevation angle of said aircraft from the elevation angle of said glide path, means for deriving in separate azimuth and elevation channels signals corresponding to those stored in said zones, said azimuth channel signals being correlated with sweep signals for display on a cathodel ray tube, means in said elevation channel comprising a monostable pulse circuit for producing a single pulse of predetermined time duration substantially equal to that of a single one of said sweep signals and corresponding to said stored elevation spot and means to mix the said single predetermined time duration pulse and signals from said azimuth channel in time relationship, and a cathode ray tube including a luminescent screen, means controlling said cathode ray tube in response to said mixed signals1 whereby a visible display is produced on said screen in which a visible spot has a position with respect to an undeiiected cathode ray tube spot position corresponding to the position of said aircraft in range and azimuth with respect to said ground station, and a visible line resultant from said single predetermined time duration pulse is distant from said undeflected cathode ray tube spot position an amount corresponding to said range pluse a quantity proportional to deviation of said elevation angle of said aircraft from the elevation angle of said glide path at said range and said visible line runs through said visible spot only when said aircraft is at the elevation angle of said glide path. g

3. A ground approach system for tracking an aircraft comprising at a ground station, radar means scanning alternately in azimuth and elevation throughout respective sectors which include the glide path to be followed in landing,

said radar means providing substantially conf tinuous information as to the range, azimuth, and elevation of an aircraft within said sectors, a storage tube having a cathode ray and storage target means for storing said information in an azimuth zone and an elevation zone distinct from each other on said target, means controlling said cathode ray in response to said radar means during scanning in azimuth to produce in said azimuth zone a stored spot whose position with respect to the undeflected position of said ray corresponds to the position of said aircraft in range and azimuth with respect to said ground station, and means controlling said cathode ray during elevation to produce in said elevation zone a stored spot whose position along a line with respect to said undeiiected position corresponds to said range plus a quantity proportional to the elevation angle of said aircraft from the elevation angle of said glide path, means for deriving in separate azimuth and elevation channels signals corresponding to those stored in said zones, said elevation channel signals being a series of pulses and both channel signals being correlated with sweep signals for display on a cathode ray tube, means in said elevation channel comprising a pulse stretching circuit connected to receive said series of pulses to stretch them to form a single pulse, a differentiator connected to receive the stretched single pulse, multivibrator means connected to receive the diiferentiated stretched pulse, and a monostable pulse circuit connected to said multivibrator means for producing a single pulse of predetermined time duration substantially equal to the time duration of a single one of said sweep signals and corresponding to said stored elevation spot, and means to mix the said last-mentioned single pulse and signals from said elevation channel in time relationship, whereby the said mixed signals correspond to those of a cathode ray tube display in which a visible spot has a position with respect to an undefiected cathode ray tube spot position corresponding to the position of said aircraft in range and azimuth with respect to said ground station, and a visible line in said display resultant from said last-mentioned single pulse is distant from said undeflected spot position an amount corresponding to said range plus a quantity proportional to deviation of said elevation angle of said aircraft from the elevation angle of said glide path at said range and said visible line runs through said display spot only when said aircraft is at the elevation angle of said glide path,

4. A ground approach system for tracking an aircraft comprising at a ground station, radar means scanningY alternately in azimuth and elevation throughout respective sectors which include the glide path to be followed in landing, said radar means providing substantially continuous information as to the range, azimuth, and elevation of an aircraft within said sectors, a storage tube having a cathode ray and storage target means for storing said information in an azimuth zone and an elevation zone distinct from each other on said target, means controlling said cathode ray in response to said radar means during scanning in azimuth to produce in said azimuth zone a stored spot whose position with respect to the undeflected position of said ray corresponds to the position of said aircraft in range and azimuth with respect to said ground station, and means controlling said cathode ray during scanning in elevation to produce in said elevation zone a stored spot whose position along a line with respect to said undeiiected position corresponds to said range plus a quantity proportional to the elevation angle of said aircraft from the elevation angle of said glide path, means for deriving in separate azimuth and elevation cha nels signals corresponding to those stored in said zones, said elevation channel signals corresponding to said elevation zone spot being a series of pulses and both channel signals being correlated with sweep signals for display on a cathode ray tube, means in said elevation channel comprising a pulse stretching circuit connected to receive said series of pulses to stretch them to form a single pulse, a differentiator connected to receive the stretched single pulse, multivibrator means connected to receive the differentiated stretched pulse, and a monostable pulse circuit connected to said multivibrator means for producing a single pulse of predetermined time duration substantially equal to the time duration of a single one of said sweep signals and corresponding to said stored elevation spot means to mix the said single predetermined time duration pulse and signals from said azimuth channel in time relationship, and ar cathode ray tube including a luminescent screen, means controlling said cathode ray tube in response to mixed Signals, whereby-avisible:display-is produced on said screen, in which a visible spot has a position with respect to an `undeiected cathode vray tube spot position corresponding to the `position of said `aircraft in. range and azimuth with .respect to said ground station, and a visible 'line resultant 4from said predetermined time duration single pulse is distant from said undeected cathode ray tube spot position an amount corresponding to said range plus a quantity propo-rtional to deviation of said elevation angle of said aircraft from the elevation angle of said glide path at said range and said visible line runsI through said visible spot only when said aircraft is at the elevation angle of said glide path.

5. A ground approach system for tracking an aircraft comprising at a ground station, radar means scanning alternately in azimuth and elevation throughout respective sectors which include the glide path to be followed in landing, said radar means providing substantially continuous information as to the range, azimuth, and elevation of an aircraft within said sectors, a storage tube having a cathode ray and storage target means for storing said information in an azimuth zone and an elevation zone distinct from each other on said target, means controlling said cathode ray in response to said radar means during scanning in azimuth to produce in said azimuth zone a stored spot whose position with respect to the undeected position of said ray corresponds to the position of said aircraft in range and azimuth with respect to said ground station, and means controlling said cathode ray during scanning in elevation to produce in said elevation zone a stored spot whose position along a line with respect to said undeflected position corresponds to said range plus a quantity proportional to the elevation angle of said aircraft from the elevation angle of said glide path, means for deriving in separate azimuth and elevation channels signals corresponding to those stored in said zones, said elevation channel signals corresponding to the elevation zone spot being a series of pulses and both said channel signals being correlated with sweep signals for display on a cathode ray tube, means in said elevation channel comprising a pulse stretching circuit connected to receive said series of pulses to stretch them to form a single pulse, a diiferentiator connected to receive the stretched single pulse, multivibrator means connected to receive the differentiated stretched pulse including a pair o-f multivibrator circuits with a common load capacitor, a monostable pulse circuit connected to said load capacitor for producing a pulse of predetermined time duration substantially equal in time duration to a single one of said sweep signals, means to apply a D. C. deflection voltage to said storage tube during one of said azimuth and elevation scans to compensate for the time displacement introduced in said elevation channel including that between said predetermined time duration pulse and an instant of time half-way between the first and last of said elevation channel series of pulses corresponding to the stored elevation spot, and means to mix the said predetermined time duration pulse and signals from said azimuth channel in time relationship, whereby the said mixed signals correspond to those of a cathode ray tube display in which a spot corresponding to said azimuth spot has a position with respect to an undeflected cathode ray tube spot position corresponding to the position of .fsaidfaircrat mirangeainiifazimuth with respect to said ground station, and a'vi-s'ible-Lline j.resultant Tf1-'om Lsaid pulse 'Eis distant from said undeectedispot `position an :amount lcorres''pond- 'ing to said range -:plus a 'quantity.proportionalito deviation of said-elevationiarrgle-'of said aircraft from the elevation angle of said glide path at said 'range and saidline runs through said display spot only when said aircraft is at the elevation angle of said glide path.

6. The system claimed in claim 2, said means controlling said cathode ray tube including a television transmitter link between said mixer and said cathode ray tube.

7. The system claimed in claim 2, said monostable pulse circuit comprising a monostable multivibrator.

8. In an electrical system having a storage tube, means to store a spot thereon, and means to read the stored spot to derive at each reading a series of impulses representing the stored spot, the combination comprising a pulse stretching circuit connected to said reading means to produce a single pulse having a time duration from the first to the last of said series, a differentiating circuit connected to differentiate the said single pulse, and other means comprising a monostable pulse circuit connected to receive said diiferentiated single pulse and to produce a derived pulse having its inception a predetermined time interval after the mean time of said differentiated single pulse.

9. The combination claimed in claim 8, said other means comprising a pair of monostable pulse circuits in addition to said rst, each one of said pair of monostable pulse circuits having a load circuit including a capacitor common to both said load circuits.

10. The combination claimed in claim 9, said first monostable pulse circuit being connected to said capacitor and responsive to produce a pulse at a predetermined voltage level across said capacitor.

1l. In an electrical system, means to derive a single pulse having its inception a predetermined time after the mean time of occurrence of a series of pulses, comprising a pulse stretching circuit to which said pulses are applied to produce a single stretched pulse having its inception and termination respectively substantially coincident with the first and last pulse of said series, a differentiating circuit connected to receive said stretched pulse, a timing circuit connected to receive the differentiating circuit output and having an output reaching a predetermined voltage V a predetermined time after the mean time of inception and termination of said stretched pulse, and means comprising a monostable pulse circuit connected to receive said timing circuit output to produce a pulse when said timing circuit output reaches said voltage value V having its inception a predetermined time after the mean time of occurrence of said stretched pulse.

12. In the electrical system claimed in claim 11, said timing circuit including a storage capacitor the voltage across which is said output voltage.

13. In the electrical system claimed in claim 11, said timing circuit including a storage capacitor, and a charging cut connected to charge said capacitor at a first rate in response to the differentiated pulse portion corresponding to the inception of said stretched pulse and at a second rate in response to the differentiated pulse pory 2,655,650 15 16 tion corresponding to the said stretched puise References Cited in the file of this patentV termination. UNITED STATES PA'I'ENTS 14. In the eleotrlcal system clalmed 1n claim Number Name Date 13, said charging circuit comprising a pair of monostable multivibrators having as a common 5 373,145 Sersiper et al' Apr' lo 1945 load capacitor said storage capacitor. 2430307 Smlth NOV' 4 1947 2,480,068 Wolff Aug. 23, 1949 JOHN' N MARSHALI r. 2,549,072 Epstein Apr. 17, 1951 

